Device for damping acoustic vibrations generated by the rotation of a prismatic body within a drum



Sept. 1, 1970 H. A. E. FRANZEN 3,526,292

DEVICE FOR DAMPING ACOUSTIC VIBRATIONS GENERATED BY THE ROTATION OF A PRISMATIC BODY WITHIN A DRUM Filed June 11, 1969 INVENTOR. hA/VS ARNE EDGAR FRANZE/V BY 0 LMW Arum; r

United States Patent US. Cl. 181-33 4 Claims ABSTRACT OF THE DISCLOSURE A rotating mirror prism Within a cylindrical drum having an opening for allowing radiation from the outside to strike the prism. The drum is made of felt carried by a wire net. The edges bounding the opening in the drum are curved outwards.

BACKGROUND OF THE INVENTION The invention relates to a device for damping acoustic vibrations caused by the rotation of a prismatic body within a cylindrical drum which is arranged concentrically with the rotational axis of the prismatic body and the wall of which is provided with at least one opening arranged so that the sides of the prismatic body pass by this opening in turn when the body rotates.

A prismatic body rotating within a drum is used for instance in the optical system of such cameras for photographing with infrared light which are combined with an oscilloscope on the picture screen of which a thermal picture of the photographed object can be directly viewed. The prismatic body forms part of the system for scanning the photographed object, and its sides are formed by or coated with mirrors. The purpose of the surrounding drum is to protect the rotating prism, but it must of course be provided with an opening through which the incoming and reflected radiation can pass.

When the prismatic body rotates within a drum provided with such an opening, the air in the interstices between the inside of the drum and the sides of the prismatic body is caused to rotate along with the prismatic body, and this air current coacts with the edges of the opening to produce a strong noise with a fundamental frequency depending on the rotational speed of the prismatic body. This noise is most irritating and disturbing to the persons handling the apparatus and may also disturb the proper function of the apparatus.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to reduce this drawback. According to the invention the wall of the drum is made of porous material which is carried by a wire net, and those edges of the wall which limit the said opening are bent outwards. Due to the fact that the edges of the opening are bent outwards, the distance between the rotating prism and the drum is increased at the most critical points. The greater distance between the prism and the drum at these edges results in a lower velocity of the air and a less response to the pressure pulsations from the rotating prism.

In order that an adequate damping shall be achieved, the material surrounding the rotating prism should be such that in the vicinity of the prism it has about the same acoustic impedance as air, i.e. it should be very porous. The porous material may consist of several layers having different porosity, the innermost layer having the greatest porosity. A suitable porous material for the 3,526,292 Patented Sept. 1, 1970 "ice purpose of the invention is felt. In order to retain the felt and to impart the required mechanical stability to the drum, the felt is preferably applied on the outside of a net of for instance thin metal wire. Such a net also contributes to the sound damping effect particularly if it has relatively fine meshes. Due to the pressure drop across the net sound energy is lost. Hereby the net will act as a sound source, but the sound emitted thereby has a higher frequency and a lower amplitude and is therefore easily absorbed by the felt.

DRAWING AND DETAILED DESCRIPTION The accompanying drawing shows schematically an embodiment of the invention.

The drawing shows in end view a hexagonal prism 1 which can rotate about the shaft 2. Prism 1 is surrounded by a drum which is open along part of its periphery. The drum is made of a wire net 3 which is indicated on the drawing by a dotted line, and a layer 4 of porous material which is applied outside the net 3. For the sake of clarity the dotted line indicating the net on the drawing has been drawn at a certain distance from the inner boundary line of the porous material 4, but actually the porous material 4 of course is in close contact with the net 3, and the net 3 can even be entirely embedded in the porous material 4.

The opening in the periphery of the drum is prefer ably of a rectangular form and has two sides parallel with the axis of the drum. These two sides are formed by the edges 5 and 6 which are bent outwards as shown in the drawing. At the edges *5 and 6 the net 3 is also bent outwards and folded back towards the inside of the drum as shown in the drawing whereby the edges 5 and 6 are given an increased mechanical stability.

The porous material 4 preferably consists of felt. It can be built up of several layers the innermost of which has the largest porosity.

A drum built up in this way is easy to form, and by tests it is easy to establish the most suitable shape of the bends at the edges of the opening so that the best sound-damping effect is achieved.

I claim:

:1. A device for damping acoustic vibrations generated by movements of air in a drum having an opening and surrounding a rotating prismatic light reflecting body having its axis of rotation concentric with the drum, characterized in that the wall of the drum is made of a porous material which is carried by a net, and that the edges bounding the said opening are bent outwards.

2. A device as claimed in claim 1, in which the porous material consists of several layers having different porosity, the innermost layer having the largest porosity.

3. A device as claimed in claim 1, in which the porous material is applied outside the net.

4. A device as claimed in claim 1, in which the porous material consists of felt.

References Cited UNITED STATES PATENTS Re. 19,862 2/1936 Adsit 350-7 XR 2,160,666 5/ 1939 McMahan 230-232 XR 3,181,646 5/1965 Edwards 181-42 3,286,914 11/1966 Baverstock 230232 3,312,3 89 4/1967 Matsui 230233 ROBERT S. WARD, In, Primary Examiner US. Cl. X.R. 3507, 285 

